This invention relates to an improved process for controlling the temperature in the regeneration zone in a fluid catalytic cracking process. In particular, it is related to a method of controlling the temperature in the fluidized dense catalyst phase of the regenerator of a fluid catalytic cracking unit (FCCU) having a single fluidized dense catalyst phase wherein coke-contaminated fluidizable catalytic cracking catalyst is contacted with an oxygen-containing regeneration gas in order to obtain a regenerated catalyst having a low carbon content.
The fluidized catalytic cracking of hydrocarbons is well-known in the prior art and may be accomplished using a variety of continuous cyclic processes which employ fluidized solids techniques. In such fluid catalytic cracking processes hydrocarbons are converted under conditions such that substantial portions of a hydrocarbon feed are transformed into desirable products such as gasoline, liquified petroleum gas, alkylation feedstocks and middle distillate blending stocks with concomitant by-product formation of an undesirable nature, such as gas and coke. When substantial amounts of coke deposition occur, reduction in catalyst activity and, particularly, catalyst selectivity results thereby deterring hydrocarbon conversion, reducing gasoline production and simultaneously increasing the production of less desired products. To overcome such catalyst deactivation through coke deposition, the catalyst is normally withdrawn from the reaction zone and passed to a stripping zone wherein entrained and absorbed hydrocarbons are initially displaced from the catalyst by means of stripping medium such as steam. The steam and hydrocarbons are removed and the stripped catalyst is passed to a regeneration zone where it is contacted with an oxygen-containing gas to effect combustion of at least a portion of the coke and thereby regenerate the catalyst. Thereafter, the regenerated catalyst is reintroduced to the reaction zone and therein contacted with additional hydrocarbons.
Generally, regeneration processes provide a regeneration zone wherein the coke-contaminated catalyst is contacted with sufficient oxygen-containing regeneration gas at an elevated temperature to effect combustion of the coke deposits from the catalyst. Most common of the regeneration processes are those wherein the contacting is effected in a fluidized dense catalyst phase in a lower portion of the regeneration zone constituted by passing the oxygen-containing regeneration gas upwardly through the regeneration zone. The space above the fluidized dense catalyst phase contains partially spent regeneration gases and catalyst entrained by the upward flowing regeneration gas. This portion of the regeneration zone is generally referred to as the dilute catalyst phase. The catalyst entrained in the dilute catalyst phase is recovered by gas solid separation cyclones located in the upper portions of the regeneration zone and is returned to the fluidized dense catalyst phase. Flue gas comprising carbon monoxide, other by-product gases obtained from the combustion of the coke deposits, inert gases such as nitrogen and any unconverted oxygen is recovered from the upper portion of the regeneration zone and a catalyst of reduced carbon content is recovered from a lower portion of the regeneration zone.
In the regeneration of catalytic cracking catalyst, particularly high activity molecular seive type cracking catalysts, it is desirable to burn a substantial amount of coke from the catalyst such that the residual carbon content of the regenerated catalyst is very low. A carbon-on-regenerated-catalyst content of about 0.15 weight percent or less is desirable. Cracking catalysts with such a reduced carbon content enable higher conversion levels within the reaction zone of the FCC unit and improved selectivity to gasoline and other desirable hydrocarbon products.
In the regeneration of catalytic cracking catalyst it is also desirable to operate the regeneration zone under conditions such that the flue gas leaving the regeneration zone have a carbon monoxide concentration of approximately 500 ppm or less so that the flue gas may be discharged into the atmosphere without additional treatment.
In order to obtain low carbon-on-regenerated-catalyst contents of about 0.15 wt.% or less, and a regeneration flue gas having a low carbon monoxide content, it is necessary to operate the fluidized dense catalyst phase of the regeneration zone at a temperature of from about 1275.degree. F. to about 1450.degree. F. and provide oxygen-containing regeneration gas in an amount sufficient to effect combustion of the coke to carbon dioxide and to provide from about 1 to about 10 mol% oxygen in the flue gas in order to reduce the carbon monoxide concentration in the flue gas to the levels herein indicated.
Whenever a regenerator is operated within the range of operating conditions herein described, it is important to control the temperature of the fluidized dense catalyst phase in the regenerator in order to maintain the desired operating conditions in the reaction zone and in order to avoid an uncontrolled afterburn in the dilute catalyst phase of the regeneration zone.
By after-burning is meant the further oxidation of carbon monoxide to carbon dioxide in the dilute catalyst phase. Whenever after-burning occurs in the dilute catalyst phase, it is generally accompanied by a substantial increase in the temperature due to the large quantities of heat liberated. In such circumstances, the dilute phase temperature may exceed about 1500.degree. F. and, in severe cases, may increase to about 1800.degree. F. or higher. Such high temperatures in the dilute catalyst phase are deleterious to the entrained catalyst present in the dilute catalyst phase and result in a permanent loss of catalytic activity, thus necessitating an inordinately high rate of catalyst addition or replacement to the process in order to maintain a desired level of catalytic activity in the hydrocarbon reaction zone. Such high temperatures are additionally undesirable because of the damage which may result to the mechanical components of the regeneration zone, particularly to cyclone separators employed to separate the entrained catalyst from the flue gas.
It is known that commonly employed catalytic cracking catalysts such as amorphous silica-alumina, silica-alumina zeolitic molecular sieves, silica-alumina zeolitic molecular sieves ion-exchanged with divalent metal ions, rare earth metal ions, etc., and mixtures thereof, are adversely affected by exposure to excessively high temperatures. At temperatures of approximately 1500.degree. F. and higher, the structure of such catalytic cracking catalyst undergo physical change, usually observeable as a reduction in the surface area with resulting substantial decrease in catalytic activity. Consequently, it is desirable to maintain the temperatures within the regeneration zone at levels below which there is any substantial physical damage to the catalyst.
Known methods for controlling the temperature of the fluidized dense catalyst phase of the regeneration zone generally include the following: adjusting the pre-heat of the oxygen-containing regeneration gas to the regeneration zone; removing heat from the fluidized dense catalyst phase by direct or indirect heat-exchange with a suitable heat-exchange medium; adjusting the oxygen-containing regeneration gas rate to control the combustion of coke within the fluidized dense catalyst phase; and adjusting the conversion level within the reaction zone of the FCCU in order to adjust the coke laydown on the spent catalyst being regenerated. All of these methods are unsatisfactory in that none provides for a method of controlling the temperature within the fluidized dense catalyst phase of the regeneration zone without requiring additional heat-exchange provisions or flue gas treatment facilities for control of carbon monoxide content while maintaining the conversion level within the reaction zone at the desired levels.